Tomographic reconstruction has been employed in conjunction with measurements of x-ray absorption to generate three-dimensional images of objects, and has been of great value in medical and other types of imaging. It has been effectively employed to image differences in density based on the stopping power of the imaged objects for the x-rays employed. Unfortunately, conventional tomographic reconstruction employed heretofore, while able to produce high-quality imaging, has not been able to produce images that reliably distinguish between different elements or isotopes.
Nuclear Resonance Fluorescence (NRF) results when a nuclear isotope is resonantly excited by photon absorption and then subsequently decays via photon emission to a lower-lying state of that nucleus. The energies of the incident photons that are resonant with a specific isotope are determined by the structure of that specific isotope and the nature of the strong nuclear interactions that bind its nucleus. These excitation energies are unique for each nuclear isotope and present a signature that identifies the presence of that specific nuclear isotope. The resulting emitted photon energies are characteristic of the specific isotope and therefore may uniquely allow the identification of the presence of that isotope in a region of space. Because the photon energies of NRF states are in the MeV energy region, the photons involved are very penetrating, allowing for deep penetration through materials.
These attributes of NRF technology enable many practical applications. For example, the isotopic detection capabilities of NRF combined with its deep penetrability make it an effective non-intrusive inspection modality for cargo containers, suitcases and packages. See U.S. Pat. No. 5,115,459, Bertozzi, “Explosives Detection Using Resonance Fluorescence of Bremsstrahlung Radiation,” U.S. Pat. No. 5,420,905, Bertozzi, “Detection of Explosives and Other Materials Using Resonance Fluorescence, Resonance Absorption, and Other Electromagnetic Processes with Bremsstrahlung Radiation,” and U.S. Pat. No. 7,120,226, Ledoux et al., “Adaptive Scanning Of Materials Using Nuclear Resonance Fluorescence Imaging,” the contents of all of which are hereby incorporated by reference.